[PATCH 5/8] powerpc: Restore FPU/VEC/VSX if previously used

[Cyril Bur <cyrilbur@gmail.com>]

Currently the FPU, VEC and VSX facilities are lazily loaded. This is not a
problem unless a process is using these facilities.

Modern versions of GCC are very good at automatically vectorising code, new
and modernised workloads make use of floating point and vector facilities,
even the kernel makes use of vectorised memcpy.

All this combined greatly increases the cost of a syscall since the kernel
uses the facilities sometimes even in syscall fast-path making it
increasingly common for a thread to take an *_unavailable exception soon
after a syscall, not to mention potentially taking the trifecta.

The obvious overcompensation to this problem is to simply always load all
the facilities on every exit to userspace. Loading up all FPU, VEC and VSX
registers every time can be expensive and if a workload does avoid using
them, it should not be forced to incur this penalty.

An 8bit counter is used to detect if the registers have been used in the
past and the registers are always loaded until the value wraps to back to
zero.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
---
 arch/powerpc/include/asm/processor.h |  2 ++
 arch/powerpc/kernel/asm-offsets.c    |  2 ++
 arch/powerpc/kernel/entry_64.S       | 55 ++++++++++++++++++++++++++++--
 arch/powerpc/kernel/fpu.S            |  4 +++
 arch/powerpc/kernel/process.c        | 66 ++++++++++++++++++++++++++++++------
 arch/powerpc/kernel/vector.S         |  4 +++
 6 files changed, 119 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index ac23308..dcab21f 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -236,11 +236,13 @@ struct thread_struct {
 #endif
 	struct arch_hw_breakpoint hw_brk; /* info on the hardware breakpoint */
 	unsigned long	trap_nr;	/* last trap # on this thread */
+	u8 load_fp;
 #ifdef CONFIG_ALTIVEC
 	struct thread_vr_state vr_state;
 	struct thread_vr_state *vr_save_area;
 	unsigned long	vrsave;
 	int		used_vr;	/* set if process has used altivec */
+	u8 load_vec;
 #endif /* CONFIG_ALTIVEC */
 #ifdef CONFIG_VSX
 	/* VSR status */
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 221d584..0f593d7 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -95,12 +95,14 @@ int main(void)
 	DEFINE(THREAD_FPSTATE, offsetof(struct thread_struct, fp_state));
 	DEFINE(THREAD_FPSAVEAREA, offsetof(struct thread_struct, fp_save_area));
 	DEFINE(FPSTATE_FPSCR, offsetof(struct thread_fp_state, fpscr));
+	DEFINE(THREAD_LOAD_FP, offsetof(struct thread_struct, load_fp));
 #ifdef CONFIG_ALTIVEC
 	DEFINE(THREAD_VRSTATE, offsetof(struct thread_struct, vr_state));
 	DEFINE(THREAD_VRSAVEAREA, offsetof(struct thread_struct, vr_save_area));
 	DEFINE(THREAD_VRSAVE, offsetof(struct thread_struct, vrsave));
 	DEFINE(THREAD_USED_VR, offsetof(struct thread_struct, used_vr));
 	DEFINE(VRSTATE_VSCR, offsetof(struct thread_vr_state, vscr));
+	DEFINE(THREAD_LOAD_VEC, offsetof(struct thread_struct, load_vec));
 #endif /* CONFIG_ALTIVEC */
 #ifdef CONFIG_VSX
 	DEFINE(THREAD_USED_VSR, offsetof(struct thread_struct, used_vsr));
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index c8b4225..46e9869 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -210,7 +210,54 @@ system_call:			/* label this so stack traces look sane */
 	li	r11,-MAX_ERRNO
 	andi.	r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
 	bne-	syscall_exit_work
-	cmpld	r3,r11
+
+	/*
+	 * This is an assembly version of checks performed in restore_math()
+	 * to avoid calling C unless absolutely necessary.
+	 * Note: In order to simplify the assembly, if the FP or VEC registers
+	 * are hot (and therefore restore_math() isn't called) the
+	 * LOAD_{FP,VEC} thread counter doesn't get incremented.
+	 * This is likely the best thing to do anyway because hot regs indicate
+	 * that the workload is doing a lot of syscalls that can be handled
+	 * quickly and without the need to touch FP or VEC regs (by the kernel).
+	 * a) If this workload is long running then this is exactly what the
+	 * kernel should be doing.
+	 * b) If this workload isn't long running then we'll soon fall back to
+	 * calling into C and the counter will be incremented regularly again
+	 * anyway.
+	 */
+	ld	r9,PACACURRENT(r13)
+	andi.	r0,r8,MSR_FP
+	addi	r9,r9,THREAD
+	lbz	r5,THREAD_LOAD_FP(r9)
+	/*
+	 * Goto 2 if !r0 && r5
+	 * The cmpb works because r5 can only have bits set in the lowest byte
+	 * and r0 may or may not have bit 13 set (different byte) but will have
+	 * a zero low byte therefore the low bytes must differ if r5 == true
+	 * and the bit 13 byte must be the same if !r0
+	 */
+	cmpb	r7,r0,r5
+	cmpldi	r7,0xff0
+#ifdef CONFIG_ALTIVEC
+	beq	2f
+
+	lbz	r9,THREAD_LOAD_VEC(r9)
+	andis.	r0,r8,MSR_VEC@h
+	/* Skip (goto 3) if r0 || !r9 */
+	bne	3f
+	cmpldi	r9,0
+	beq 3f
+#else
+	bne 3f
+#endif
+2:	addi    r3,r1,STACK_FRAME_OVERHEAD
+	bl	restore_math
+	ld	r8,_MSR(r1)
+	ld	r3,RESULT(r1)
+	li	r11,-MAX_ERRNO
+
+3:	cmpld	r3,r11
 	ld	r5,_CCR(r1)
 	bge-	syscall_error
 .Lsyscall_error_cont:
@@ -592,8 +639,8 @@ _GLOBAL(ret_from_except_lite)
 
 	/* Check current_thread_info()->flags */
 	andi.	r0,r4,_TIF_USER_WORK_MASK
-#ifdef CONFIG_PPC_BOOK3E
 	bne	1f
+#ifdef CONFIG_PPC_BOOK3E
 	/*
 	 * Check to see if the dbcr0 register is set up to debug.
 	 * Use the internal debug mode bit to do this.
@@ -608,7 +655,9 @@ _GLOBAL(ret_from_except_lite)
 	mtspr	SPRN_DBSR,r10
 	b	restore
 #else
-	beq	restore
+	addi	r3,r1,STACK_FRAME_OVERHEAD
+	bl	restore_math
+	b	restore
 #endif
 1:	andi.	r0,r4,_TIF_NEED_RESCHED
 	beq	2f
diff --git a/arch/powerpc/kernel/fpu.S b/arch/powerpc/kernel/fpu.S
index 2117eac..b063524 100644
--- a/arch/powerpc/kernel/fpu.S
+++ b/arch/powerpc/kernel/fpu.S
@@ -130,6 +130,10 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
 	or	r12,r12,r4
 	std	r12,_MSR(r1)
 #endif
+	/* Don't care if r4 overflows, this is desired behaviour */
+	lbz	r4,THREAD_LOAD_FP(r5)
+	addi	r4,r4,1
+	stb	r4,THREAD_LOAD_FP(r5)
 	addi	r10,r5,THREAD_FPSTATE
 	lfd	fr0,FPSTATE_FPSCR(r10)
 	MTFSF_L(fr0)
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 441d9e5..c602b67 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -366,6 +366,53 @@ void giveup_all(struct task_struct *tsk)
 	msr_check_and_clear(msr_all_available);
 }
 
+void restore_math(struct pt_regs *regs)
+{
+	unsigned long msr;
+
+	if (!current->thread.load_fp
+#ifdef CONFIG_ALTIVEC
+		&& !current->thread.load_vec)
+#else
+		)
+#endif
+		return;
+
+	msr = regs->msr;
+	msr_check_and_set(msr_all_available);
+
+	/*
+	 * Only reload if the bit is not set in the user MSR, the bit BEING set
+	 * indicates that the registers are hot
+	 */
+#ifdef CONFIG_PPC_FPU
+	if (current->thread.load_fp && !(msr & MSR_FP)) {
+		load_fp_state(&current->thread.fp_state);
+		msr |= MSR_FP | current->thread.fpexc_mode;
+		current->thread.load_fp++;
+	}
+#endif
+#ifdef CONFIG_ALTIVEC
+	if (current->thread.load_vec && !(msr & MSR_VEC) &&
+			cpu_has_feature(CPU_FTR_ALTIVEC)) {
+		load_vr_state(&current->thread.vr_state);
+		current->thread.used_vr = 1;
+		msr |= MSR_VEC;
+		current->thread.load_vec++;
+	}
+#endif
+#ifdef CONFIG_VSX
+	if (!(msr & MSR_VSX) && (msr & (MSR_FP | MSR_VEC)) == (MSR_FP | MSR_VEC)) {
+		current->thread.used_vsr = 1;
+		msr |= MSR_VSX;
+	}
+#endif
+
+	msr_check_and_clear(msr_all_available);
+
+	regs->msr = msr;
+}
+
 void flush_all_to_thread(struct task_struct *tsk)
 {
 	if (tsk->thread.regs) {
@@ -806,17 +853,9 @@ void restore_tm_state(struct pt_regs *regs)
 
 	msr_diff = current->thread.ckpt_regs.msr & ~regs->msr;
 	msr_diff &= MSR_FP | MSR_VEC | MSR_VSX;
-	if (msr_diff & MSR_FP) {
-		msr_check_and_set(MSR_FP);
-		load_fp_state(&current->thread.fp_state);
-		msr_check_and_clear(MSR_FP);
-		regs->msr |= current->thread.fpexc_mode;
-	}
-	if (msr_diff & MSR_VEC) {
-		msr_check_and_set(MSR_VEC);
-		load_vr_state(&current->thread.vr_state);
-		msr_check_and_clear(MSR_VEC);
-	}
+
+	restore_math(regs);
+
 	regs->msr |= msr_diff;
 }
 
@@ -977,6 +1016,11 @@ struct task_struct *__switch_to(struct task_struct *prev,
 		batch = this_cpu_ptr(&ppc64_tlb_batch);
 		batch->active = 1;
 	}
+
+	/* Don't do this on a kernel thread */
+	if (current_thread_info()->task->thread.regs)
+		restore_math(current_thread_info()->task->thread.regs);
+
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
 	return last;
diff --git a/arch/powerpc/kernel/vector.S b/arch/powerpc/kernel/vector.S
index 162d0f7..038cff8 100644
--- a/arch/powerpc/kernel/vector.S
+++ b/arch/powerpc/kernel/vector.S
@@ -91,6 +91,10 @@ _GLOBAL(load_up_altivec)
 	oris	r12,r12,MSR_VEC@h
 	std	r12,_MSR(r1)
 #endif
+	/* Don't care if r4 overflows, this is desired behaviour */
+	lbz	r4,THREAD_LOAD_VEC(r5)
+	addi	r4,r4,1
+	stb	r4,THREAD_LOAD_VEC(r5)
 	addi	r6,r5,THREAD_VRSTATE
 	li	r4,1
 	li	r10,VRSTATE_VSCR
-- 
2.6.2